Method of preparing an electrode for a capacitive deionization device, an electrode for a capacitive deionization device, and a capacitive deionization device having the electrode

ABSTRACT

A method of preparing an oxidized electrode for a capacitive deionization device, the method including electrochemically oxidizing an electrode including a hydrophobic active material to produce the oxidized electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No.10-2010-0000577, filed on Jan. 5, 2010, and all benefits accruingtherefrom under 35 U.S.C. §119, the content of which in its entirety isherein incorporated by reference.

BACKGROUND

1. Field

The present disclosure relates to a method of preparing an electrode fora capacitive deionization device, an electrode for a capacitivedeionization device, and a capacitive deionization device having theelectrode.

2. Description of the Related Art

Tap water supplied to homes contains hardness components, e.g., variouswater-hardening minerals such as calcium, although the contents thereofvary according to the region where the home is located. In particular,in Europe where large amounts of limestone components contactunderground water, the hardness of tap water is significant.

Unwanted and undesirable scaling easily occurs in a heat exchanger of ahome appliance or on an inner wall of a boiler when hard water, whichcontains high concentrations of hardness components, is used therein,and thus the energy efficiency of the device is significantly reduceddue to the scaling. In addition, hard water is unsuitable for washingdue to the difficulty of producing a lather with the hard water. Methodsfor overcoming such problems associated with the use of hard waterinclude (i) removing the scaling with chemicals, and (ii) chemicallysoftening hard water using ion exchange resins, wherein after use thecontamination in the ion exchange resin may be removed using a largeamount of high-concentration salt water, so that the ion exchange resinmay be reused. However, such methods are inconvenient and causeenvironmental damage. Thus, there is a demand for a technology to moresimply soften hard water in an environmentally friendly manner.

Capacitive deionization (“CDI”) is a technology for removing an ionicmaterial from a medium by adsorbing the ionic material onto a surface ofan electrode having nano-sized pores by applying a first voltage to theelectrode. To regenerate the electrode, a second voltage opposite inpolarity to the first voltage is applied to the electrode, which may bea carbon electrode, so as to remove the adsorbed ionic material, and theionic material is discharged with water. CDI may operate withoutchemicals to regenerate the carbon electrode and may operate without anion exchange resin, a filter, or a membrane. Also, CDI may improvecapacitance of the medium, which may be water, without discharginghardness components, such as Ca²⁺ or Mg²⁺, or harmful ions, such as Cr.

In CDI, when a direct current (“DC”) voltage having a low potentialdifference with respect to the medium is applied to a carbon electrodewhile a medium, i.e., an electrolyte solution containing dissolved ions,flows through a flow path and contacts the carbon electrode, anions areabsorbed and concentrated in a negative electrode, and cations areabsorbed and concentrated in a positive electrode. Accordingly, whenapplication of the DC voltage is stopped, the concentrated anions andcations are desorbed from the negative electrode and positive electrode.

The carbon electrode desirably has a low electrical resistance and alarge specific surface area, and thus, the carbon electrode may bemanufactured by binding an activated carbon with polytetrafluoroethlyene(“PTFE”), or the carbon electrode may be manufactured by carbonizing aresorcinol formaldehyde resin and then performing a complicated dryingprocess, thereby obtaining a carbon electrode having a plate-like shape.

Commercial electrodes for CDI are usually in the form of a sheet and aremanufactured by binding an activated carbon with PTFE. The activatedcarbon has a large specific surface area and numerous pores, and thus,has high processing capacity when the activated carbon is used as anactive material for a CDI electrode.

In addition, high surface area graphite (“HSAG”) has a relatively largespecific surface area and is inexpensive, and thus, may be applied to anactive material in spite of a high degree of graphitization.

However, an electrode including the carbonaceous material has lowhydrophilicity, and thus has low wettability in CDI influent water.Also, reduction of the wettability of the electrode is caused byrepulsion between the hydrophobic carbonaceous material and the influentwater, thereby decreasing the capacitance and deionization rates of theelectrode. Thus there remains a need for an improved CDI electrodematerial.

SUMMARY

Provided is a method of preparing an electrode for a capacitivedeionization device, the method including electrochemically oxidizing anelectrode including a hydrophobic active material.

Provided is an electrode for a capacitive deionization device, theelectrode including an electrochemically oxidized active material.

Provided is a capacitive deionization device including the electrode.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description.

According to an aspect, a method of manufacturing an oxidized electrodefor a capacitive deionization device includes: electrochemicallyoxidizing an electrode including a hydrophobic active material toproduce the oxidized electrode.

The electrode may further include a binder and a conducting agent.

The method may further include disposing a powdered hydrophobic activematerial.

The hydrophobic active material may include a carbonaceous material.

The carbonaceous material may include at least one selected from thegroup consisting of activated carbon, carbon nanotube (“CNT”),mesoporous carbon, activated carbon fiber, graphite, and graphite oxide.

The graphite may include high surface area graphite (“HSAG”).

The electrochemical oxidization of the electrode may be performed in anelectrochemical cell including the electrode, a counter electrode, anelectrolyte solution in which the electrode and the counter electrodeare immersed, and a separator which electrically insulates the electrodefrom the counter electrode, wherein the electrode is used as a positiveelectrode, and the counter electrode is used as a negative electrode inthe electrochemical cell.

The electrode and the counter electrode may be the same or differentfrom each other.

The electrolyte solution may be an acidic, alkaline, or neutralsolution.

An amount of electric charge charged in the electrochemical cell duringthe electrochemically oxidizing the electrode may be in the range ofabout 20 to about 30,000 coulombs per gram, based on the weight of anactive material of the electrode.

A contact angle between the electrode and a hydrophilic electrolytesolution may be decreased by the electrochemical oxidization of theelectrode.

The method may further include washing the electrochemically oxidizedelectrode with a cleaning solution.

According to another aspect of the present invention, an electrode for acapacitive deionization device includes an electrochemically oxidizedactive material.

The electrode for a capacitive deionization device may further includean acidic functional group.

The amount of the acidic functional group per unit weight of theelectrode may be about 0.7 to about 5 millimoles per gram.

The acidic functional group may include at least one selected from thegroup consisting of a carboxyl group (—COOH), a carboxylate group(—COO⁻), and a phenol group.

According to another aspect of the present invention, a capacitivedeionization device includes the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of an exemplary embodiment ofan electrochemical cell used in a method of preparing an electrode for acapacitive deionization device;

FIG. 2 is a schematic cross-sectional view of an exemplary embodiment ofa capacitive deionization device including an electrode;

FIGS. 3A and 3B are photographs of exemplary embodiments ofelectrochemically oxidized electrodes which are prepared according to anembodiment and are in contact with hard water, and FIG. 3C is aphotograph of an exemplary embodiment of an electrode which is notelectrochemically oxidized and is in contact with hard water;

FIG. 4 is a graph illustrating ionic conductivity of effluent water(microsiemens per centimeter, μS/cm) versus treatment time (seconds,sec) when influent water is treated using a comparative capacitivedeionization device, and describes how to calculate a charge amount anda discharge amount;

FIGS. 5A and 5B are graphs of ionic conductivity (microsiemens percentimeter, μS/cm) versus treatment time (seconds, sec) respectivelyshowing ionic conductivity of effluent water according to treatment timewhen deionization of influent water is conducted using an exemplaryembodiment of a capacitive deionization device including an exemplaryembodiment of an electrode or using a capacitive deionization deviceincluding a comparative electrode;

FIGS. 6A and 6B are graphs of charge amount (microsiemens seconds percentimeter, μm·sec/cm) versus charge cycle number respectively showingcharge amount with respect to number of charge cycles when deionizationof influent water is conducted using a capacitive deionization deviceincluding an exemplary embodiment of an electrode or using a capacitivedeionization device including a comparative electrode; and

FIGS. 7A and 7B are graphs respectively showing discharge rate (inverseseconds, sec⁻¹) versus time (seconds, sec) when deionization of influentwater is conducted using an exemplary embodiment of a capacitivedeionization device including an exemplary embodiment of an electrode orusing a capacitive deionization device including a comparativeelectrode.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to the like elements throughout. In this regard, thepresent embodiments may have different forms and should not be construedas being limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below by referring to the figures toexplain aspects of the present description.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer, orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer, or section discussed belowcould be termed a second element, component, region, layer, or sectionwithout departing from the teachings of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising,” or“includes” and/or “including” when used in this specification, specifythe presence of stated features, regions, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, regions, integers, steps,operations, elements, components, and/or groups thereof.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother elements as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in one of the figures is turned over, elementsdescribed as being on the “lower” side of other elements would then beoriented on “upper” sides of the other elements. The exemplary term“lower,” can therefore, encompasses both an orientation of “lower” and“upper,” depending on the particular orientation of the figure.Similarly, if the device in one of the figures is turned over, elementsdescribed as “below” or “beneath” other elements would then be oriented“above” the other elements. The exemplary terms “below” or “beneath”can, therefore, encompass both an orientation of above and below.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

Hereinafter, a method of preparing an electrode for a capacitivedeionization device, an electrode prepared using the method, and acapacitive deionization device including the electrode are described infurther detail with reference to the accompanying drawings.

The method of preparing an electrode for a capacitive deionizationdevice includes electrochemically oxidizing an electrode having ahydrophobic active material. By electrochemically oxidizing theelectrode, the hydrophobicity of the hydrophobic active materialcontained in the electrode decreases and the hydrophilicity of theactive material increases, which will be further described below.

The hydrophobic active material may include a carbonaceous material. Thecarbonaceous material may include at least one selected from the groupconsisting of activated carbon, carbon nanotube (“CNT”), mesoporouscarbon, activated carbon fiber, graphite, and graphite oxide. Inaddition, the graphite may include high surface area graphite (‘HSAG”).The term “HSAG” as used herein refers to graphite having a specificsurface area equal to or greater than about 30 square meters per gram(m²/g).

The electrode may further include a binder and a conducting agent. In anembodiment, the electrode may be prepared using a powdered hydrophobicactive material. For example, the electrode may be prepared by adding apowdered hydrophobic active material and a conducting agent to a bindersuspension, stirring the mixture, kneading the stirred mixture, andpress molding the kneaded stirred mixture.

The binder may include styrene butadiene rubber (“SBR”),carboxymethylcellulose (“CMC”), polytetrafluoroethlyene (“PTFE”), or thelike, or a combination comprising at least one of the foregoing.

The conducting agent may include carbon black, vapor grown carbon fiber(“VGCF”), graphite, or the like, or a combination comprising at leastone of the foregoing.

Hereinafter, the electrochemically oxidizing the electrode will befurther described in detail with reference to FIG. 1.

FIG. 1 is a schematic cross-sectional view of an exemplary embodiment ofan electrochemical cell 10 used in a method of preparing a workingelectrode 13 a for a capacitive deionization device.

Referring to FIG. 1, the electrochemical cell 10 may include a reactionvessel 11, an electrolyte solution 12, a working electrode 13 a, and acounter electrode 13 b, which are immersed in the electrolyte solution12, and a separator 14 which electrically insulates the workingelectrode 13 a from the counter electrode 13 b.

In the electrochemical cell 10, the working electrode 13 a may be usedas a positive electrode, and the counter electrode 13 b may be used as anegative electrode. That is, if a positive (+) electrical potential isapplied to the working electrode 13 a and a negative (−) electricalpotential is applied to the counter electrode 13 b in theelectrochemical cell 10, the working electrode 13 a, which is a positiveelectrode, is electrochemically oxidized. In an embodiment, whether theworking electrode 13 a is electrochemically oxidized or not may beidentified by stirring (e.g., contacting) the working electrode 13 awith a cleaning solution, such as deionized water, to wash the workingelectrode 13 a, and titrating the resulting deionized water solutionwith an alkaline solution, and/or measuring a contact angle between theworking electrode 13 a and a hydrophilic electrolyte solution (e.g.,hard water). Thus, if the hydrophobic active material contained in theworking electrode 13 a is electrochemically oxidized, an acidicfunctional group, which may include oxygen, may be present on thesurface of the active material, and the amount of the acidic functionalgroup may be measured by titrating the solution collected after washingthe working electrode 13 a with an alkaline solution. Also, a contactangle between the working electrode 13 a and the hydrophilic electrolytesolution may be smaller than a contact angle between the workingelectrode 13 a and the hydrophilic electrolyte solution before theworking electrode 13 a is electrochemically oxidized. The term “contactangle” as used herein refers to an angle between a free surface of aliquid (e.g., a hydrophilic electrolyte solution such as hard water) anda tangent plane of a solid (e.g., an electrode) where the liquid meetsthe solid. The contact angle is determined by a cohesive force betweenliquid molecules and an adhesive force between the surface of the liquidand the surface of the solid. As such, while not wanting to be bound bytheory, it is understood that if the amount of the acidic functionalgroup formed on the surface of the working electrode 13 a increases, thecontact angle between the working electrode 13 a and the hydrophilicelectrolyte solution decreases, and thus the hydrophilicity of thesurface of the working electrode 13 a is improved and the wettability ofthe working electrode 13 a to the hydrophilic electrolyte solution(e.g., hard water) increases. In addition, the specific surface area ofthe working electrode 13 a is proportionate to the amount of the acidicfunctional group that exists on the surface of the working electrode 13a, e.g., the amount of the alkaline solution used for titrating theacidic functional group (see, for example, Carbon 37 (1999), 85-96, C.U. Pittman et al).

The amount of the acidic functional group per unit weight of theelectrochemically oxidized electrode may be about 0.7 to about 5millimoles per gram (mmol/g), specifically 1 to 4 mmol/g, morespecifically about 3 mmol/g. When the amount of the acidic functionalgroup is within this range, the wettability of the electrode to thehydrophilic electrolyte solution may be improved.

The acidic functional group may include at least one oxygen-containingfunctional group selected from the group consisting of a carboxyl group(—COOH), a carboxylate group (—COO⁻), and a phenol group.

The working electrode 13 a and the counter electrode 13 b may be thesame or may be different from each other. For example, the workingelectrode 13 a and the counter electrode 13 b may each independentlyinclude activated carbon. For example, the working electrode 13 a mayinclude HSAG and the counter electrode 13 b may include activatedcarbon.

The electrolyte solution may be an acidic solution such as an aqueousH₂SO₄ solution, an alkaline solution such as an aqueous KOH solution, ora neutral solution such as an aqueous KCl solution.

In addition, the amount of electric charge charged in theelectrochemical cell 10 in the electrochemically oxidizing the electrodemay be about 20 to about 30,000 coulombs per gram (C/g), specificallyabout 100 to about 10,000 C/g, more specifically about 1,000 to about5,000 C/g, based on a weight of an active material of the electrode. Ifthe amount of the electric charge is within the foregoing range, thehydrophilicity of the electrode may be improved with reduced energycosts and within a shortened period of time.

The working electrode 13 a, which is electrochemically oxidized asdisclosed above, may be washed with a cleaning solution such asdistilled water, and then may be used as a positive electrode, such aspositive electrode 101 a of FIG. 2 and/or a negative electrode, such asnegative electrode 101 b of the capacitive deionization device 100 ofFIG. 2. In an embodiment, the electrochemically oxidized positiveelectrode 101 a and/or the negative electrode 101 b of FIG. 2 has highwettability to the hydrophilic electrolyte solution and high specificsurface area, as further disclosed above. Accordingly, capacitance andlife-span characteristics of the electrode are improved, anddeionization rates of influent water, such as hard water, andregeneration rates of the electrode increase, thereby improving theperformance of the capacitive deionization device 100 of FIG. 2.

The electrochemical oxidization of the electrode for a capacitivedeionization device may be conducted using the electrochemical cell 10of FIG. 1 and/or the capacitive deionization device 100 of FIG. 2 (see,for example, Examples 1 to 6).

FIG. 2 is a schematic cross-sectional view of an exemplary embodiment ofa capacitive deionization device 100 including a positive electrode 101a and/or negative electrode 101 b.

Referring to FIG. 2, the capacitive deionization device 100 may includea pair of electrodes, specifically a positive electrode 101 a and anegative electrode 101 b, a pair of current collectors, specifically apositive current collector 102 a and a negative current collector 102 b,and a separator 103. In an embodiment, the positive electrode 101 a andthe negative electrode 101 b may respectively be disposed on oppositesides of the separator 103, and each of the positive and negativecurrent collectors 102 a and 102 b may respectively be disposed at aside of the positive electrode 101 a and the negative electrode 101 bwhich is opposite to the separator 103.

At least one of the positive electrode 101 a and the negative electrode101 b may be an electrode which is electrochemically oxidized accordingto the method disclosed above. For example, the positive electrode 101 amay include activated carbon which is not electrochemically oxidized,and the negative electrode 101 b may include electrochemically oxidizedHSAG. Alternatively, the positive electrode 101 a and the negativeelectrode 101 b may respectively include an electrochemically oxidizedactivated carbon.

The influent water (e.g., hard water), which flows into the capacitivedeionization device 100 at a first end of the separator 103, isdeionized while passing through the capacitive deionization device 100,and thus is changed into treated water (e.g., soft water), which isdischarged at a second end of the separator 103.

A power supply (“PS”) may be electrically connected to the positive andnegative current collectors 102 a and 102 b, respectively. Thus, thepositive and negative current collectors 102 a and 102 b may provide anelectrical path to supply electric charge to the positive and negativeelectrodes 101 a and 101 b during charging, i.e., when the influentwater is deionized, and to discharge the electric charge accumulated inthe positive and negative electrodes 101 a and 101 b during discharging,i.e., when the positive and negative electrodes 101 a and 101 b areregenerated. The positive and negative current collectors 102 a and 102b may be a carbon plate, a carbon paper, a metal plate, a metal mesh, ametal foam, or the like, and may include aluminum, nickel, copper,titanium, stainless steel, iron, or the like, or a combinationcomprising at least one of the foregoing.

The separator 103 provides a flow path between the pair of stackedpositive and negative electrodes 101 a and 101 b, and blocks electricalcontact between the positive and negative electrodes 101 a and 101 b andtheir corresponding current collectors, the positive and negativecurrent collectors 102 a and 102 b, respectively. The separator 103 mayinclude, for example, an acrylic fiber, a polyethylene film, apolypropylene film, or a combination comprising at least one of theforegoing.

Hereinafter, the operation and effects of the capacitive deionizationdevice 100 are disclosed in further detail.

First, a process of deionizing influent water (also referred to as acharging process) is performed as follows. In an embodiment, theinfluent water is to be deionized and also functions as an electrolytesolution of the capacitive deionization device 100.

When the power supply PS applies a direct current (“DC’) voltage to thepositive and negative current collectors 102 a and 102 b, the influentwater flows into the capacitive deionization device 100 at a first endof the separator 103. The positive electrode 101 a, which iselectrically connected to a positive (+) terminal of the power supplierPS via the positive current collector 102 a, is positively charged, andthe negative electrode 101 b, which is electrically connected to anegative (−) terminal of the power supplier PS via the negative currentcollector 102 b, is negatively charged. Thus the positive and negativeelectrodes are polarized. Referring to FIG. 2, the positive electrode101 a, which is positively charged, and the negative electrode 101 b,which is negatively charged, face each other, and the separator 103 isdisposed therebetween. Accordingly, anions, which may include harmfulions such as Cl⁻ and may be contained in the influent water, areadsorbed onto the positive electrode 101 a, which is positively charged,and cations, which may including hardness components such as Ca²⁺ andMg²⁺ and may be contained in the influent water, are adsorbed onto thenegative electrode 101 b, which is negatively charged. As the processingtime passes, the anions and cations, which are dissolved in the influentwater, are adsorbed and accumulated in the positive electrode 101 a andthe negative electrode 101 b. Accordingly, the influent water, whichpasses through the capacitive deionization device 100, is deionized andturned into treated water. Moreover, at least a portion of the harmfulions, which may be included in the influent water, are removed. However,after additional processing time, a surface of the active materialincluded in the positive and negative electrodes 101 a and 101 b may becovered (e.g., saturated) with the adsorbed cations or anions, and thus,a deionizing efficiency of the influent water may gradually decrease.The deionizing efficiency may be determined by measuring the ionicconductivity of treated water (e.g., soft water) flowing out from thecapacitive deionization device 100 over time. In other words, when theionic conductivity of the treated water is low, the amount of removedcations and anions may be large, and thus, the deionizing efficiency maybe high. On the contrary, when the ionic conductivity of the treatedwater is high, the amount of the removed cations and anions may besmall, and thus, the deionizing efficiency may be low.

Thus, if the ionic conductivity of the treated water is equal to orgreater than a selected value, it may be desirable to regenerate thepositive and negative electrodes 101 a and 101 b. In an embodiment, whenthe power supplied to the capacitive deionization device 100 is stoppedand the capacitive deionization device 100 is electrically shorted, soas to discharge the capacitive deionization device 100, the positive andnegative electrodes 101 a and 101 b may become unpolarized, and the ionswhich are adsorbed onto the active material of the positive and negativeelectrodes 101 a and 101 b may be desorbed. Thus, the active surfaces ofthe positive and negative electrodes 101 a and 101 b may be restored.

Although the capacitive deionization device 100 of FIG. 2 includes oneseparator, a pair of electrodes, and a pair of current collectors, thedisclosed device is not limited thereto. For example, an exemplaryembodiment of a capacitive deionization device may be any of the devicesdisclosed in Korean Patent Application Nos. 2008-0123154 and2009-0077161, which are herein incorporated by reference in theirentirety, further including at least one electrochemically oxidizedelectrode as disclosed above.

Hereinafter, one or more embodiments of the present disclosure will bedisclosed in further detail with reference to the following examples.However, these examples are not intended to limit the purpose and scopeof the one or more embodiments of the disclosure.

EXAMPLES Examples 1 to 6 Preparation of Capacitive Deionization CellElectrode Manufacture

Manufacture of Activated Carbon Electrode

A 45 gram (g) quantity of activated carbon (PC, available from Osaka GasCo., Ltd.), 5 g of carbon black (Super P, available from Timcal), and8.3 g of an aqueous suspension of 60% by weight ofpolytetrafluoroethylene (“PTFE”), and 100 g of polyvinyl alcohol wereput into a stirring vessel, kneaded, and then press-molded. Theresulting mixture was dried in an oven at 80° C. for 2 hours, at 120° C.for 1 hour, and at 200° C. for 1 hour to complete the manufacture of anactivated carbon electrode, referred to below as a “PC electrode.”

Manufacture of HSAG Electrode

A 45 g quantity of high surface area graphite (HSAG, available fromTimcal), 5 g of carbon black (Super P, available from Timcal), and 8.33g of an aqueous suspension of 60% by weight of polytetrafluoroethylene(“PTFE”), and 100 g of polyvinyl alcohol were put into a stirringvessel, kneaded, and then press-molded. Then, the resulting mixture wasdried in an oven at 80° C. for 2 hours, at 120° C. for 1 hour, and at200° C. for 1 hour to complete the manufacture of an HSAG electrode.

Electrochemically Oxidized HSAG Electrode Manufacture

First, each of the PC electrode and the HSAG electrode prepared abovewas cut to prepare 2 pieces, each having of the dimensions 10centimeters (cm)×10 cm and an area of 100 square centimeters (cm²).

Second, the two pieces of the PC electrode and the two pieces of theHSAG electrode were put into distilled water and werevacuum-impregnated.

Third, an electrochemical cell was prepared by sequentially stacking acurrent collector (graphite foil), one piece of the two pieces of HSAGelectrode prepared above, a separator (polyester fiber: manufactured bySefar), one piece of the two pieces of PC electrode prepared above, andthe current collector (graphite foil).

Fourth, an aqueous 0.5 M H₂SO₄ solution or an aqueous 0.5 M KCl solutionwas added to each electrochemical cell, as shown in Table 1 below, asthe electrolyte solution, and each electrochemical cell was assembled.

Fifth, both electrodes of the electrochemical cells were charged using aconstant current-constant voltage regime. Specifically the cells werecharged using a constant current of 2 A applied to both electrodes untila voltage between the electrodes reached 3 V, and then the cells werecharged by applying a constant voltage of 3 V to the electrodes, therebycharging the electrodes to have an electric charge of 200 to 3,000 C/gas shown in Table 1 below. During the charging, the HSAG electrode wasused as a positive electrode, and the PC electrode was used as anegative electrode.

Sixth, the electrochemically oxidized HSAG electrode was washed withdistilled water.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Electrolyte H₂SO₄ H₂SO₄ H₂SO₄ KCl KCl KCl solution (0.5M aqueoussolution) Amount of 3,000 1,000 300 3,000 1,000 200 electric charge(C/g)

Capacitive Deionization Cell Manufacture

A capacitive deionization cell was prepared by sequentially stacking acurrent collector (graphite foil), the electrochemically oxidized HSAGelectrodes prepared above, a separator (polyester fiber: manufactured bySefar), the other piece of the PC electrodes prepared above, and acurrent collector (graphite foil). In the capacitive deionization cell,during the deionizing, the PC electrode was used as a positiveelectrode, and the HSAG electrode was used as a negative electrode.

Comparative Example Preparation of Capacitive Deionization Cell

An HSAG electrode which was not electrochemically oxidized and a PCelectrode were prepared in the same manner as in Examples 1 to 6, andthen a capacitive deionization cell including the PC electrode as apositive electrode, and the HSAG electrode, which was notelectrochemically oxidized, as a negative electrode during thedeionizing, was prepared in the same manner as in Examples 1 to 6.

EVALUATION EXAMPLES Evaluation Example 1 Evaluation of Degree ofOxidation of Electrochemically Oxidized HSAG Electrode

The electrochemically oxidized HSAG electrodes prepared according toExamples 1 and 4, and the HSAG electrode which was not electrochemicallyoxidized and prepared according to the Comparative Example, were eachadded to deionized water, and the deionized water stirred at 150revolutions per minute (“rpm”) for 4 days to desorb the functionalgroups attached on the surface of the HSAG electrodes. Then, thedeionized water obtained after washing the HSAG electrodes was titratedusing an aqueous 0.01 M NaOH solution. The amount of NaOH used in thetitration is shown in Table 2 below.

TABLE 2 Comparative Example 1 Example 4 Example Amount of NaOH used inthe 2.213 1.105 0.383 titration (mmol/g)

Referring to Table 2, the electrochemically oxidized HSAG electrodesprepared according to Examples 1 and 4 had more acidic functional groupsthan the HSAG electrode which was not electrochemically oxidized andprepared according to the Comparative Example, and thus it wasdetermined that the electrochemically oxidized HSAG electrodes havehigher hydrophilicity.

Evaluation Example 2 Evaluation of Contact Angle of ElectrochemicallyOxidized HSAG Electrode

The electrochemically oxidized HSAG electrodes prepared according toExamples 1 and 4 and the HSAG electrode which was not electrochemicallyoxidized and prepared according to the Comparative Example werecontacted with hard water according to IEC 60734. FIGS. 3A to 3C arephotographs of the flat HSAG electrodes in contact with a droplet-likehard water, and contact angles were measured using a contact anglemeasuring device (DSA 10, available from Kruss) and listed in Table 3below.

FIGS. 3A and 3B are photographs of the electrochemically oxidizedelectrodes which are prepared according to Examples 1 and 4 and are incontact with hard water, and FIG. 3C is a photograph of the HSAGelectrode which is not electrochemically oxidized, prepared according tothe Comparative Example and is in contact with hard water. The contactangles shown in FIGS. 3A to 3C are provided in Table 3.

TABLE 3 Comparative Example 1 Example 4 Example Contact angle (°) 96 94122

Referring to FIGS. 3A to 3C, and Table 3, the electrochemically oxidizedHSAG electrodes prepared according to Examples 1 and 4 had a smallercontact angle than the HSAG electrode which was not electrochemicallyoxidized and was prepared according to the Comparative Example. Thus itwas determined that the electrochemically oxidized HSAG electrodes havehigher hydrophilicity than the HSAG electrode which was notelectrochemically oxidized.

Evaluation Example 3 Evaluation of Life-Span Characteristics of CellElectrode

The capacitive deionization cells prepared according to Examples 1 to 6and the Comparative Example were operated under the conditions disclosedbelow. Then, the ionic conductivity of the effluent water (i.e., treatedwater during charging or waste water during discharging) according tothe treatment time (Examples 1 and 4, and Comparative Example), chargeamount with respect to the number of charge cycles (Examples 1 to 6, andComparative Example), and discharge rates over time (Examples 1 and 4,and Comparative Example) thereof were measured, and the results areshown in FIGS. 5A and 5B, 6A and 6B, and 7A and 7B.

First, each cell was operated at room temperature while the electrodeswere immersed in the electrolyte solution.

Second, hard water according to IEC 60734 was used as influent water,and the flow rate of hard water was adjusted to 30 milliliters perminute (mL/min).

Third, each cell was charged using a constant voltage of 2.0 V for 10minutes (“min”), and then discharged for 20 minutes by electricallyshorting the electrodes.

The charge amount may be calculated using Equation 1 below as shown inFIG. 4. FIG. 4 is a graph illustrating ionic conductivity of effluentwater versus treatment time when influent water is treated using acomparative capacitive deionization device, and describes how tocalculate a charge amount and a discharge amount. The charge amount isin proportion to the amount of ions in the influent water removed by theelectrochemical cell.

Q _(a)=(λ_(i) t _(c))−Q _(c)  Equation 1

In Equation 1, Q_(a) is the charge amount, λ_(i) is the ionicconductivity of the influent water measured before passing through thecell, t_(c) is the charging time, and Q_(c) is determined by integratingthe ionic conductivity of the effluent water over the actual chargingtime.

In addition, the discharge amount may be calculated using Equation 2below as shown in FIG. 4. The discharge amount is in proportion to thedesorption ratio of ions from an electrode.

Q _(b) =Q _(d)−(λ_(i) t _(d))  Equation 2

In Equation 2, Q_(b) is the discharge amount, Q_(d) is determined byintegrating ionic conductivity of the effluent water over the actualdischarging time, λ_(i) is the ionic conductivity of the influent watermeasured before passing through cell, and t_(d) is the discharging time.

FIGS. 5A and 5B are graphs respectively showing ionic conductivity ofeffluent water according to treatment time when deionization of influentwater is conducted using an exemplary embodiment of a capacitivedeionization device including an exemplary embodiment of an electrode orusing a capacitive deionization device including a comparativeelectrode. In FIGS. 5A and 5B, the concave peaks represent charge peaks,and the convex peaks represent discharge peaks.

Referring to FIGS. 5A and 5B, the ionic conductivity of the treatedwater more rapidly decreased during the charging and the ionicconductivity of the waste water more rapidly increased during thedischarging in Examples 1 and 4 when compared to the ComparativeExample. Based on these results, it is identified that a larger amountof ions are removed from the influent water, and faster deionizationrates and faster electrode regeneration rates are obtained in Examples 1and 4 than the Comparative Example.

FIGS. 6A and 6B are graphs respectively showing charge amount withrespect to the number of charge cycles when deionization of influentwater is conducted using an exemplary embodiment of a capacitivedeionization device including an exemplary embodiment of an electrode orusing a capacitive deionization device including a comparativeelectrode.

Referring to FIGS. 6A and 6B, each of the capacitive deionizationdevices prepared according to Examples 1 to 6 has a larger charge amountthan the capacitive deionization device prepared according to theComparative Example. As the charge amount increases, the deionizationrate and the deionization amount increase. Furthermore, because thecharge amount just slightly decreases as the number of charge cycleincreases in the capacitive deionization devices prepared according toExamples 4 to 6, it is determined that the electrodes of Examples 4 to 6have improved life-span characteristics.

The charge amount of the first charge cycle shown in FIGS. 6A and 6B isshown in Table 4 below.

TABLE 4 Comparative Example 1 Example 2 Example 3 Example 4 Example 5Example 6 Example Charge amount 2,257,000 2,257,000 1,512,000 1,967,0001,595,000 1,381,000 1,199,000 (uS · sec/cm) Relative charge 188 188 126164 133 115 100 amount (%) (based on the charge amount of theComparative Example)

Referring to Table 4, the charge amount of the first charge cycle inExamples 1 to 6 was about 1.15 to about 1.88 times of that in theComparative Example.

FIGS. 7A and 7B are graphs respectively showing discharge rates versustime when deionization of influent water is conducted using an exemplaryembodiment of a capacitive deionization device including an exemplaryembodiment of an electrode or using a capacitive deionization deviceincluding a comparative electrode. In this regard, “discharge rate” asused herein refers to a value obtained by dividing a discharge amountper second by the total discharge amount.

Referring to FIGS. 7A and 7B, each of the capacitive deionizationdevices prepared according to Examples 1 and 4 has a faster dischargerate than the capacitive deionization device prepared according to theComparative Example. As the discharge rate increases, the regenerationrate of the electrode increases, and the amount of the influent waterconsumed for the electrode regeneration process decreases.

It should be understood that the exemplary embodiments described hereinshould be considered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

1. A method of manufacturing an oxidized electrode for a capacitivedeionization device, the method comprising: electrochemically oxidizingan electrode comprising a hydrophobic active material to produce theoxidized electrode.
 2. The method of claim 1, wherein the electrodefurther comprises a binder and a conducting agent.
 3. The method ofclaim 2, wherein the electrode is prepared using a powdered hydrophobicactive material.
 4. The method of claim 1, wherein the hydrophobicactive material comprises a carbonaceous material.
 5. The method ofclaim 4, wherein the carbonaceous material comprises at least oneselected from the group consisting of activated carbon, carbon nanotube,mesoporous carbon, activated carbon fiber, graphite, and graphite oxide.6. The method of claim 5, wherein the graphite comprises high surfacearea graphite.
 7. The method of claim 1, wherein the electrochemicaloxidization of the electrode is performed in an electrochemical cellcomprising the electrode, a counter electrode, an electrolyte solutionin which the electrode and the counter electrode are immersed, and aseparator which electrically insulates the electrode from the counterelectrode, wherein the electrode is used as a positive electrode, andthe counter electrode is used as a negative electrode in theelectrochemical cell.
 8. The method of claim 7, wherein the electrodeand the counter electrode are the same or different from each other. 9.The method of claim 7, wherein the electrode and the counter electrodeare the same.
 10. The method of claim 7, wherein the electrode and thecounter electrode are different from each other.
 11. The method of claim7, wherein the electrolyte solution is an acidic, alkaline, or neutralsolution.
 12. The method of claim 7, wherein the electrolyte solution isan acidic solution.
 13. The method of claim 7, wherein the electrolytesolution is an alkaline solution.
 14. The method of claim 7, wherein theelectrolyte solution is a neutral solution.
 15. The method of claim 7,wherein an amount of electric charge charged in the electrochemical cellduring the electrochemically oxidizing the electrode is about 20 toabout 30,000 coulombs per gram, based on a weight of an active materialof the electrode.
 16. The method of claim 7, wherein a contact anglebetween the electrode and a hydrophilic electrolyte solution isdecreased by the electrochemical oxidization of the electrode.
 17. Themethod of claim 1, further comprising washing the electrochemicallyoxidized electrode with a cleaning solution.
 18. An electrode for acapacitive deionization device, the electrode comprising: anelectrochemically oxidized active material.
 19. The electrode of claim18, further comprising an acidic functional group on the electrode. 20.The electrode of claim 19, wherein the amount of the acidic functionalgroup per unit weight of the electrode is about 0.7 to about 5millimoles per gram.
 21. The electrode of claim 19, wherein the acidicfunctional group comprises at least one selected from the groupconsisting of a carboxyl group, a carboxylate group, and a phenol group.22. A capacitive deionization device comprising the electrode of claim18.